Correct Answer: An employee receives instructions from only one supervisor

Solution : The principle of unity of command states that each employee should receive instructions from and be accountable to only one supervisor. This helps to avoid confusion, minimize conflict, and ensure clear lines of authority within the organization. Option C is an example of this principle, as the employee receives instructions from only one supervisor, thereby minimizing the possibility of conflicting or confusing instructions from multiple supervisors.

Hence, Option D is correct.